The present invention relates to a thermal analysis system and method for detection and quantitative measurement of an analyte undergoing a thermally-induced, energy event having an associated enthalpy change. Thermal analysis generally has been characterized by a two-thermocouple system. The present invention, in employing a single thermocouple, a thermopile, or other temperature sensor and in providing for automated data acquisition, simplifies analysis and provides for increased sensitivity.
Thermal analysis is generally characterized as a quantitative analytical method that is used, for example, to thermodynamically detect the concentration of an analyte in a sample. For many compounds, the normal room temperature phase may not be stable over the entire solid or liquid state temperature range. As the temperature of such compounds is increased from room temperature, a competing phase may have lower free energy and the compound will accordingly experience a phase change. Concomitant with or independent of the phase change, the analyte may also experience a decomposition or a chemical reaction with another compound. The phase change or chemical reaction produces an endothermic or exothermic enthalpy change, i.e., a liberation or absorption of heat, which can be measured and from which the concentration of the compound in the sample can be determined.
Thermal analysis traditionally has been characterized by a differential method employing two thermocouples joined in series. A first thermocouple is disposed in thermal adjacency with a first sample containing an analyte whose concentration is to be determined. A second thermocouple is disposed in thermal adjacency with an second, inert sample substantially free of the compound contained in the first sample. The samples are then placed in a furnace and the temperatures of the samples are raised to beyond the phase transition temperature of the compound in first sample. The temperature or energy difference between the samples is measured upon heating and/or cooling to derive a thermal analysis curve having a peak representative of the enthalpy change of the analyte in the first sample. From peak height or peak area, the concentration of the analyte in the first sample can be determined.
Two-thermocouple differential thermal analysis, however, is inherently disadvantaged in its sensitivity to minute temperature changes in the sample. As a result of differences in heat capacity, heat leakage, density and thermal conductivity between the sample and the reference material, considerable baseline drift can occur and cause significant interpretation problems, particularly when quantitative results are required.
To remedy the perceived limitations in the two-thermocouple thermal analysis method, investigators have proposed a single-thermocouple thermal analysis system. Sheffield et al. have described a single-thermocouple thermal analysis method wherein the sample itself is used as the reference. See "Single Thermocouple Differential Thermal Analysis With Application to Quantitative Low Level Detection of Free Crystalline Quartz," Thermochimica Acta, 32 (1979) 45-52. The method disclosed entails disposing a thermocouple connected to a digital multimeter in thermal adjacency with a sample contained in a furnace. A reference junction isothermally maintained in a reference bath is series coupled to the sample thermocouple. The sample is heated to a specified temperature, and voltage readings are manually recorded as a function of time as the sample experiences Newtonian cooling. Voltage-time data outside of the transition region is fitted to a least squares polynomial and the residuals in the transition region are summed at discrete data points to yield a value proportional to the transition enthalpy.
MacMillan, U.S. Pat. No. 3,360,993, describes a single-thermocouple technique for determining the transition temperature of compounds by measuring the difference in EMF (V) between a single thermocouple located in a sample and an ice-point-reference thermocouple. The difference derived then is differentiated with respect to time (t) by an analog servo system having an output proportional to dV/dt which is automatically recorded as a function of sample temperature.
Although the single-thermocouple techniques described in the prior art do afford some advantages over the traditional two-thermocouple method, there nevertheless remains a need for an automated thermal analysis system with increased sensitivity to detect and quantify, for example, minute quantities of quartz, cristobalite and the like contained in ceramic or other materials. Such a need has been spurred by evidence that airborne particles of crystalline silica, e.g., quartz, cristobalite, tridymite, and the like, pose respiratory and possible carcinogenic health hazards even at low concentrations. In 1987, the International Agency for Research on Cancer (IARC) declared crystalline silica a possible human carcinogen, i.e., category 2A. See IARC Monographs on the Evaluation of the Carcinogenic Risk of Chemical to Humans, Vol. 42, Silica and Some Silicates, World Health Organization, International Agency for Research on Cancer, Lyons, France, 1987. In the U.S., the Occupational Safety and Health Administration (OSHA) enforces a permissible exposure limit (PEL) of 0.1 mg/m.sup.3 and regulates substances containing.gtoreq.0.1% crystalline silica. See U.S. Occupational Safety and Health Administration Toxic and Hazardous Substances, 29 C.F.R. 1910, 1200, U.S. Government Printing Office, Washington, D.C., 1989.
Detection of crystalline silica at the 0.1% regulatory threshold has, however, challenged industrial mineral producers, mineral importers and exporters, analytical equipment manufacturers, and analysts. X-ray diffraction traditionally has been the analytical method of choice for quantitative determination of mineral phases. However, most X-ray diffraction instruments, except those of the most recent generation, have lacked the sensitivity to achieve detection limits of 0.1% or less. Moreover, X-ray diffraction is disadvantaged in that many of the common rock forming minerals, i.e., feldspar, kaolin, barite, have diffraction lines that interfere with one or more of the major quartz diffraction lines. X-ray diffraction is also matrix dependent, making it difficult to develop a single analytical method that can be applied to a variety of silica-containing materials. In addition, X-ray results can be influenced by the non-crystalline short range order that can occur regularly in SiO.sub.2 -based materials and may, accordingly, indicate crystallinity in samples even where no phase transitions occur.
Moreover, even at levels above the 0.1% regulatory threshold, detection of crystalline silica and the like is of commercial importance in that a volume change also accompanies the phase transition. For example, length changes of about 0.2% can occur when quartz undergoes its transition. For cristobalite, the length change can be as much as 1.1%. Such volumetric changes can produce considerable stresses within an article of manufacture which, in turn, can result in cracking and consequent premature failure of the article. Consequently, it may be seen that there has existed and remains a need for an automated system capable of quantitatively measuring amounts of crystalline silica and the like.